The selection of target optical properties for a filter will vary significantly, depending upon the intended application. For example, U.S. Pat. No. 5,071,206 to Hood et al., which is assigned to the assignee of the present patent document, describes a filter arrangement which may be used for automotive, housing and office windows. The Hood et al. patent states that the arrangement of layers provides color correction, heat reflectivity and infrared reflectivity. In comparison, the desired properties of a coating for a plasma display panel (PDP) may be somewhat different. Published U.S. Patent Application No. 2006/0055308 to Lairson et al., which is also assigned to the assignee of the present patent document, describes factors which are considered in the design of an optical filter for a PDP. The identified factors are the degree of neutrality of transmitted color, the level of reflected light, the color shift with changes in the incidence angle of a viewer, and the transmission levels of infrared and electromechanical radiation. Unfortunately, whether designing an optical filter for coating windows or coating a PDP, there are tradeoffs among the different factors. Thus, modifying a filter to increase conditions with respect to one desired property may conflict with maintaining a target level for another property.
FIG. 1 is one possible arrangement of layers to provide a filter for a plasma display panel, which includes a module or separate glass sheet 10. The Etalon filter 12 is first formed on a polyethylene terephthalate (PET) substrate 14 that is then affixed to the glass sheet by a layer of adhesive 16. Because a plasma display generates infrared radiation and electromagnetic interference (EMI) that must be controlled in accordance with legislated regulations, the filter layers 12 are designed to reduce infrared and EMI from the display. Etalon filters based on multiple silver layers are used to screen infrared wavelengths and electromagnetic waves. Interference between adjacent silver layers can be tuned to cause resonant transmission in the visible region, while providing desirable screening. The above-referenced patent to Hood et al. describes a suitable sequence of layers.
FIG. 1 also includes an antireflection (AR) layer stack 18 that was originally formed on a second PET substrate 20. Antireflection layer stacks are well known in the art. A second adhesive layer 22 secures the PET substrate 20 to the other elements of FIG. 1.
While the PDP filter 12 reduces infrared transmission and EMI from the display, the filter must also be cosmetically acceptable and must enable good fidelity in the viewing of displayed images. Thus, the transmissivity of the filter should be high in the visual region of the light spectrum and should be relatively colorless, so as not to change the color rendering of the plasma display. Further, a general expectation exists that displays should be low in reflectance and that the reflected color be bluish to slightly reddish.
Color can be expressed in a variety of fashions. In the above-cited Hood et al. patent, color is expressed in the CIE La*b*1976 color coordinate system and in particular the ASTM 308-85 method. Using this method, a property is shown by values for a* and b* near 0. Generally, consumers expect that computer displays will appear either neutral or slightly bluish in color. Referring briefly to the La*b* coordinate system shown in FIG. 2, this generally yields the expectation that reflected a* (i.e., Ra*) lies in the range of −2 to approximately 10, and reflected b* (i.e., Rb*) lies in the range −40 to approximately 2. This expectation is shown by dashed lines 23.
Users of large information displays generally expect minimal change in reflected color with changes in the viewing angle. Any color change is distracting when a display is viewed from a close distance, where the color of the display appears to change across the surface. Since plasma display panels are intrinsically large, due to the large number of pixels required for imaging and the large pixel size, the need for reduced color travel with viewing angle is heightened. In particular, it is objectionable if the “red-green” component of color, Ra*, changes substantially with angle. Changes along the other axis, Rb*, are generally less of an issue when the display has large reflected negative Rb* (i.e., strong blue reflected color) at normal incidence.
As previously noted, different factors regarding the design of PDP filters may conflict. The same is true in the design of a filter for a window. Generally, controlling reflected color competes with EM screening capability. Typical silver etalon filters work to screen infrared rays primarily by reflecting the rays. Infrared radiation is relatively close in wavelength to red and is therefore difficult to effectively control while simultaneously obtaining low reflection in the red region of the spectrum (i.e., 620-700 nm). The problem is particularly acute for plasma displays, where it is desirable to shield from Xe emissions at 820 nm and 880 nm while maintaining high transmissivity in the red region of the spectrum.
Controlling reflection within the red region of the light spectrum is rendered even more difficult by the need for a low sheet resistance in the PDP filter 12. Attempts have been made to balance the goals of maximizing red transmission and minimizing sheet resistance. U.S. Pat. No. 6,102,530 to Okamura et al. describes an optical filter for plasma displays, where the filter has a sheet resistance of less than 3 ohms/square. Generally, a sheet resistance of less than 1.5 ohms/square is required to meet Federal Communication Commission (FCC) Class B standard, even for PDP sets having the highest luminance efficiencies. A sheet resistance of less than 1.4 ohms/square is preferred. Copper wire mesh PDP EMI filters having a sheet resistance of 0.1 to 0.2 ohms/square are often used to provide Class B compatibility.
The requirement for lower sheet resistance increases the color problem for etalon EMI filters. The transmission bandwidth of the filter becomes narrower as the conductive layers become thicker, resulting in both an increase in the red reflection and a loss of color bandwidth in transmission.
There is a conflict between the tendency of Etalon filters to show red reflection at different viewing angles and the generally expected appearance of consumer products. This is known from the design of automotive windshields, where a disagreeable “purple” appearance is produced by reflections of clouds from certain windshields. This objectionable reflection limits the thickness of the conductive layers used in such filters.
FIG. 2 illustrates the difficulty with a four silver layer coating designed for a PDP. The plot 24 shows color as a function of viewing angle from normal incidence to 60 degrees. The four silver layer coating may have an acceptable sheet resistance and may have a total silver thickness of 45 nm to provide an acceptable color appearance at normal incidence. However, as the illustration shows, when the coating is viewed at 60 degrees, the reflected light is strongly red, with Ra* of approximately 30. In addition, there is a large color shift with incidence angle, which creates an apparent color difference across the screen for a large screen viewed at a close distance. Thus, despite the suitability of the coating for some Class B EMI applications, this coating may be considered cosmetically unacceptable.
The above-cited reference to Lairson et al. describes a filter arrangement having a number of advantages over prior techniques. The filter arrangement includes at least five metallic layers, such as silver or silver alloy layers, that are spaced apart by dielectric layers. There may be five metallic layers and six dielectric layers. The reference states that the dielectric layers may be indium oxide or a combination of indium oxide and titanium oxide.
While prior art approaches to providing optical filter arrangements show continuing advancements in achieving target levels with respect to desired properties, further advancements are sought. In the ideal, such advancements can be achieved while simultaneously reducing the cost.